A cellulose ester film, a polycarbonate film, a poly cyclic olefin film, and so on are widely used as an optical film for liquid crystal displays.
It is required for an optical film that its transparency should be optically high and also its birefringence should be low. Especially, in recent years, the size of a liquid crystal display becomes lager and the luminance becomes higher and higher. In connection with these, an improvement in front contrast is demanded more severely than ever before.
Since the transparency of a cellulose ester film is optically high and also its birefringence is low, it has been mainly used as a protective film (hereinafter, referred to as a polarizing plate protective film) of a polarization film of a liquid crystal display. A polycarbonate film and a poly cyclic olefin film have mainly been used as an optical compensation film for adjusting a retardation.
In order to improve the front contrast, an improvement in transmittance of each member constituting a liquid crystal display has been examined continuously. However, also the improvement in transmittance has been continuously examined about an optical compensation at the cell side of a polarizing plate without exception.
Moreover, in order to control a retardation and its wavelength dispersion characteristic, an optical compensation film has been usually used in combination with plural sheets of optical films.
However, since the combination of the plural sheets of optical films has a large load in productivity in terms of combination precision and an increase in the number of processes, an optical compensation technology with a small number of sheets has been studied.
For example, in Liquid crystal “various functional films for a liquid crystal display element” in the special edition of 9th Volume No. 4 (2005) in the journal of Japanese Liquid Crystal Society, a technology to make it into one sheet with a polycarbonate film and a poly cyclic olefin film has been proposed. However, even if such a technology is used, as an optical compensation film which serves as a polarizing plate protective film, it has an insufficient pasting ability with polyvinyl alcohol being a polarization film, and a polarizing plate protective film consisting of a cellulose ester film has been recognized to be an indispensable optical film in a liquid crystal display even now.
Then, it has been studied to provide a function as an optical compensation film to the cellulose ester film which is excellent as this polarizing plate protective film.
Basically, since a cellulose ester film has the circumstances where it has been used as a polarizing plate protective film due to its low birefringence characteristic, it may be not easy to provide the function.
In order to acquire a desired retardation value, a technique to add a compound having a retardation increasing effect to a cellulose ester film- and to further stretch the film is proposed (Patent Documents 1, 2, 3, 4, 5), but there are problems that the wavelength dispersion characteristic does no stabilize over time and the permeability of the film is deteriorated by the stretching.
If the wavelength dispersion characteristic is insufficient, phenomena such as light leakage from a polarizing plate (especially light leakage with an elapse of time), color tone fluctuation of an image on a liquid crystal display and front contrast deterioration occur.
Therefore, it has been desired eagerly to provide the wavelength dispersion characteristic stable over time and a desired retardation value simultaneously to a cellulose ester film.
The transmittance deterioration of a film is presumed to be an increase in haze (dispersion factor), and the phenomenon of deteriorating the front contrast occurs.
Therefore, it has been desired eagerly to provide a desired retardation value and a decrease of haze simultaneously to a cellulose ester film.
Patent Document 1: Japanese Patent Unexamined Publication No. 200-111914
Patent Document 2: Japanese Patent Unexamined Publication No. 2002-131538
Patent Document 3: Japanese Patent Unexamined Publication No. 2006-299171
Patent Document 4: Japanese Patent Unexamined Publication No. 2006-154803
Patent Document 5: Japanese Patent Unexamined Publication No. 2006-265382
Non Patent document: Japanese Liquid Crystal Society Journal Liquid Crystal “Various functional films for liquid crystal display elements” Special edition Vol. 9 No. 4 (2005)